MV2.array(0.).fill_value is NaN!

In the `TransientVariable.__new__ method`, the following code assign a NaN value to the fill_value attribute:

``` python

       fill_value = numpy.array(fill_value).astype(dtype),
```

So, for instance:

``` python
>>> numpy.isnan(MV2.array(0.).fill_value)
True
```
